Thursday, September 6, 2012

The Earth Mantle

Mantle (geology)

        The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is a rocky shell about 2,900 km (1,800 mi) thick that constitutes about 84% of Earth's volume. It is predominantly solid and encloses the iron-rich hot core, which occupies about 15% of Earth's volume. Past episodes of melting and volcanism at the shallower levels of the mantle have produced a thin crust of crystallized melt products near the surface, upon which we live. Information about structure and composition of the mantle either result from geophysical investigation or from direct geoscientific analyses on Earth mantle derived xenoliths.
      Two main zones are distinguished in the upper mantle: the inner asthenosphere composed of plastic flowing rock about 200 km thick, and the lowermost part of the lithosphere composed of rigid rock about 50 to 120 km thick. A thin crust, the upper part of the lithosphere, surrounds the mantle and is about 5 to 75 km thick.
          In some places under the ocean the mantle is actually exposed on the surface of the Earth. 


         The mantle is divided into sections which are based upon results from seismology. These layers (and their depths) are the following: the upper mantle (starting at the Moho, or base of the crust around 7 to 35 km downward to 410 km), the transition zone (410–660 km), the lower mantle (660–2891 km), and anomalous core-mantle boundary with a variable thickness (on average ~200 km thick).
      The top of the mantle is defined by a sudden increase in seismic velocity, which was first noted by Andrija Mohorovičić in 1909; this boundary is now referred to as the "Mohorovičić discontinuity" or "Moho". The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, an irregular layer with a maximum thickness of perhaps 200 km. Below the lithosphere the upper mantle becomes notably more plastic. In some regions below the lithosphere, the seismic velocity is reduced; this so-called low-velocity zone (LVZ) extends down to a depth of several hundred km. Inge Lehmann discovered a seismic discontinuity at about 220 km depth; although this discontinuity has been found in other studies, it is not known whether the discontinuity is ubiquitous. The transition zone is an area of great complexity; it physically separates the upper and lower mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous. The D" layer at the core-mantle boundary separates the mantle from the core.

      The mantle differs substantially from the crust in its mechanical properties which is the direct consequence of chemical composition change (expressed as different mineralogy). The distinction between crust and mantle is based on chemistry, rock types, rheology and seismic characteristics. The crust is a solidification product of mantle derived melts, expressed as various degrees of partial melting products during geologic time. Partial melting of mantle material is believed to cause incompatible elements to separate from the mantle, with less dense material floating upward through pore spaces, cracks, or fissures, that would subsequently cool and solidify at the surface. Typical mantle rocks have a higher magnesium to iron ratio and a smaller proportion of silicon and aluminium than the crust. This behavior is also predicted by experiments that partly melt rocks thought to be representative of Earth's mantle.
           Mantle rocks shallower than about 410 km depth consist mostly of olivinepyroxenesspinel-structure minerals, and garnet; typical rock types are thought to be peridotitedunite (olivine-rich peridotite), and eclogite. Between about 400 km and 650 km depth, olivine is not stable and is replaced by high pressure polymorphs with approximately the same composition: one polymorph is wadsleyite (also called beta-spinel type), and the other is ringwoodite (a mineral with the gamma-spinel structure). Below about 650 km, all of the minerals of the upper mantle begin to become unstable. The most abundant minerals present, the silicate perovskites, have structures (but not compositions) like that of the mineral perovskite followed by the magnesium/iron oxide ferropericlase. The changes in mineralogy at about 400 and 650 km yield distinctive signatures in seismic records of the Earth's interior, and like the moho, are readily detected using seismic waves. These changes in mineralogy may influence mantle convection, as they result in density changes and they may absorb or release latent heat as well as depress or elevate the depth of the polymorphic phase transitions for regions of different temperatures. The changes in mineralogy with depth have been investigated by laboratory experiments that duplicate high mantle pressures, such as those using the diamond anvil.

                                          Composition of Earth's mantle in weight percent
            Element         Amount                         Compound                          Amount
                O                            44.8
                Si                            21.5                                           SiO2                                           46                
                Mg                          22.8                                         MgO                                           37.8
                Fe                              5.8                                          FeO                                               7.5
                Al                              2.2                                           Al2O3                                          4.2
                Ca                             2.3                                           CaO                                              3.2
               Na                              0.3                                         Na2O                                           0.4
               K                                0.03                                       K2O                                             0.04
           Sum                    99.7                             Sum                               99.1         
          The inner core is solid, the outer core is liquid, and the mantle solid/plastic. This is because of the relative melting points of the different layers (nickel-iron core, silicate crust and mantle) and the increase in temperature and pressure as depth increases. At the surface both nickel-iron alloys and silicates are sufficiently cool to be solid. In the upper mantle, the silicates are generally solid (localised regions with small amounts of melt exist); however, as the upper mantle is both hot and under relatively little pressure, the rock in the upper mantle has a relatively low viscosity. In contrast, the lower mantle is under tremendous pressure and therefore has a higher viscosity than the upper mantle. The metallic nickel-iron outer core is liquid because of the high pressure and temperature. As the pressure exponentially increases, the nickel-iron inner core becomes solid because the melting point of iron increases dramatically at these high pressures.
         In the mantle, temperatures range between 500 to 900 °C (932 to 1,652 °F) at the upper boundary with the crust; to over 4,000 °C(7,230 °F) at the boundary with the core. Although the higher temperatures far exceed the melting points of the mantle rocks at the surface (about 1200 °C for representative peridotite), the mantle is almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents melting, because the temperature at which melting begins (the solidus) increases with pressure.
          This figure is a snapshot of one time-step in a model of mantle convection. Colors closer to red are hot areas and colors closer to blue are cold areas. In this figure, heat received at the core-mantle boundary results in thermal expansion of the material at the bottom of the model, reducing its density and causing it to send plumes of hot material upwards. Likewise, cooling of material at the surface results in its sinking.
        Because of the temperature difference between the Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle. Hot material upwells, while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones. Locations on the surface that lie over plumes are predicted to have high elevation (because of the buoyancy of the hotter, less-dense plume beneath) and to exhibit hot spotvolcanism. The volcanism often attributed to deep mantle plumes is alternatively explained by passive extension of the crust, permitting magma to leak to the surface (the "Plate" hypothesis).

        The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to be an integral part of the motion of plates. Plate motion should not be confused with continental drift which applies purely to the movement of the crustal components of the continents. The movements of the lithosphere and the underlying mantle are coupled since descending lithosphere is an essential component of convection in the mantle. The observed continental drift is a complicated relationship between the forces causing oceanic lithosphere to sink and the movements within Earth's mantle.
       Although there is a tendency to larger viscosity at greater depth, this relation is far from linear and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core.The mantle within about 200 km above the core-mantle boundary appears to have distinctly different seismic properties than the mantle at slightly shallower depths; this unusual mantle region just above the core is called D″ ("D double-prime"), a nomenclature introduced over 50 years ago by the geophysicist Keith BullenD″ may consist of material from subducted slabs that descended and came to rest at the core-mantle boundary and/or from a new mineral polymorph discovered in perovskite called post-perovskite.
          Earthquakes at shallow depths are a result of stick-slip faulting; however, below about 50 km the hot, high pressure conditions ought to inhibit further seismicity. The mantle is considered to be viscous and incapable of brittle faulting. However, in subduction zones, earthquakes are observed down to 670 km. A number of mechanisms have been proposed to explain this phenomenon, including dehydration, thermal runaway, and phase change. The geothermal gradient can be lowered where cool material from the surface sinks downward, increasing the strength of the surrounding mantle, and allowing earthquakes to occur down to a depth of 400 km and 670 km.
       The pressure at the bottom of the mantle is ~136 GPa (1.4 million atm). Pressure increases as depth increases, since the material beneath has to support the weight of all the material above it. The entire mantle, however, is thought to deform like a fluid on long timescales, with permanent plastic deformation accommodated by the movement of point, line, and/or planar defects through the solid crystals comprising the mantle. Estimates for the viscosity of the upper mantle range between 1019 and 1024 Pa·s, depending on depth,[20] temperature, composition, state of stress, and numerous other factors. Thus, the upper mantle can only flow very slowly. However, when large forces are applied to the uppermost mantle it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries.

          Exploration of the mantle is generally conducted at the seabed rather than on land because of the relative thinness of the oceanic crust as compared to the significantly thicker continental crust.
        The first attempt at mantle exploration, known as Project Mohole, was abandoned in 1966 after repeated failures and cost over-runs. The deepest penetration was approximately 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 ft) below the sea floor from the ocean drilling vessel JOIDES Resolution.
           On 5 March 2007, a team of scientists on board the RRS James Cook embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, mid-way between the Cape Verde Islands and the Caribbean Sea. The exposed site lies approximately three kilometres beneath the ocean surface and covers thousands of square kilometres. A relatively difficult attempt to retrieve samples from the Earth's mantle was scheduled for later in 2007. The Chikyu Hakken mission attempted to use the Japanese vessel 'Chikyu' to drill up to 7,000 m (23,000 ft) below the seabed. This is nearly three times as deep as preceding oceanic drillings.
         A novel method of exploring the uppermost few hundred kilometres of the Earth was recently proposed, consisting of a small, dense, heat-generating probe which melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks.[25] The probe consists of an outer sphere of tungsten about one metre in diameter with a cobalt-60interior acting as a radioactive heat source. It was calculated that such a probe will reach the oceanic Moho in less than 6 months and attain minimum depths of well over 100 km in a few decades beneath both oceanic and continental lithosphere.
         Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago.

                See also
                Core–mantle boundary
            Mohorovičić discontinuity
            Lehmann discontinuity
            Post-perovskite phase transition
            Mantle convection
            Mesosphere (mantle)
            Mantle xenoliths


                                                                                                                                      Hồ Đình Hải

See more pictures about the earth mantle

The Earth Core

I-The inner core of the Earth

     The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about 1,220 km (760 mi) in radius, or about 70% that of the Moon. It is believed to consist of an ironnickel alloy, and may have a temperature similar to the Sun's surface, approximately 5700 K (5430 °C).
        The existence of a solid inner core distinct from the liquid outer core was discovered in 1936 by seismologist Inge Lehmann using observations of earthquake-generated seismic waves that partly reflect from its boundary and can be detected by sensitive seismographs on the Earth's surface. This boundary is referred as Bullen discontinuity or sometimes Lehmann discontinuity.
            Later (1940) it was conjectured that this inner core was solid iron, and its rigidity was confirmed in 1971.
          The outer core was believed to be liquid due to its inability to transmit elastic shear waves; only compressional waves are observed to pass through it. The solidity of the inner core has been difficult to establish because the elastic shear waves that are expected to pass through it are very weak and difficult to detect because they also must travel through the outer core. Dziewonski and Gilbert established the consistency of this hypothesis using normal modes of vibration of Earth caused by large earthquakes. Recent claims of detections of inner core transmitted shear waves were initially controversial but are now gaining acceptance.
        Based on the abundance of chemical elements in the solar system, the theory of planetary formation, and other chemical constraints regarding the remainder of Earth's volume, the inner core is composed primarily of a nickel–iron alloy referred to as Nife: 'Ni' for nickel, and 'Fe' for ferrum or iron. Because the inner core is more dense (12.8 ~ 13.1) g⁄cm³ than pure iron or nickel, even under heavy pressures, it's believed that the remaining part of the core is composed of goldplatinum and other siderophile elements in quantity enough to coat Earth's surface for 0.45 m (1.5 feet). The relative abundance of precious metals and other heavy elements in respect to Earth's crust is explained with the theory of iron catastrophe, an event which occurred before the first eon during the accretion of early Earth.
       The temperature of the inner core can be estimated using experimental and theoretical constraints on the melting temperature of impure iron at the pressure (about 330 GPa) of the inner core boundary, yielding estimates of 5,700 K (5,430 °C; 9,800 °F). The range of pressure in Earth's inner core is about 330 to 360 gigapascals (3,300,000 to 3,600,000 atm), and iron can only be solid at such high temperatures because its melting temperature increases dramatically at these high pressures (see the Clausius–Clapeyron relation).
        J. A. Jacobs was the first to suggest that the inner core is freezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years). Prior to the inner core's formation, the entire core was molten, and the age of the inner core is thought to lie between 2–4 billion years. Because it is younger than the age of Earth (about 4.5 billion years), the inner core cannot be a primordial feature inherited during the formation of the solar system.
         Little is known about how the inner core grows. Because it is slowly cooling, many scientists expected that the inner core would be homogeneous. It was even suggested that Earth's inner core might be a single crystal of iron; However, this is at odds with the observed degree of disorder inside the inner core. Seismologists have revealed that the inner core is not completely uniform and contains large-scale structures that seismic waves pass more rapidly through than others. The surface of the inner core exhibits rapid variations in properties at scales at least as small as 1 km. This is puzzling, since lateral temperature variations along the inner core boundary are known to be extremely small (this conclusion is confidently constrained by magnetic field observations). Recent discoveries suggest that the solid inner core itself is composed of layers, separated by a transition zone about 250 to 400 km thick. If the inner core grows by small frozen sediments falling onto its surface, then some liquid can also be trapped in the pore spaces and some of this residual fluid may still persist to some small degree in much of its interior.
         Because the inner core is not rigidly connected to Earth's solid mantle, the possibility that it rotates slightly faster or slower than the rest of Earth has long been entertained. In the 1990s, seismologists made various claims about detecting this kind of super-rotation by observing changes in the characteristics of seismic waves passing through the inner core over several decades, using the aforementioned property that it transmits waves faster in some directions. Estimates of this super-rotation are around one degree of extra rotation per year, although others have concluded it is rotating more slowly than the rest of Earth by a similar amount.
          Growth of the inner core is thought to play an important role in the generation of Earth's magnetic field by dynamo action in the liquid outer core. This occurs mostly because it cannot dissolve the same amount of light elements as the outer core and therefore freezing at the inner core boundary produces a residual liquid that contains more light elements than the overlying liquid. This causes it to become buoyant and helps drive convection of the outer core. The existence of the inner core also changes the dynamic motions of liquid in the outer core as it grows and may help fix the magnetic field since it is expected to be a great deal more resistant to flow than the outer core liquid (which is expected to be turbulent).

        Speculation also continues that the inner core might have exhibited a variety of internal deformation patterns. This may be necessary to explain why seismic waves pass more rapidly in some directions than in others. Because thermal convection alone appears to be improbable, any buoyant convection motions will have to be driven by variations in composition or abundance of liquid in its interior. S. Yoshida and colleagues proposed a novel mechanism whereby deformation of the inner core can be caused by a higher rate of freezing at the equator than at polar latitudes, and S. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time.
        There is an East–West asymmetry in the inner core seismological data. There is a model which explains this by differences at the surface of the inner core – melting in one hemisphere and crystallization in the other.

II-Outer core of the Earth

          The outer core of the Earth is a liquid layer about 2,266 km (1,408 mi) kilometers thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies 2,890 km (1,800 mi) beneath the Earth's surface. The transition between the inner core and outer core is located approximately 5,150 km (3,200 mi) beneath the Earth's surface.
       The temperature of the outer core ranges from 4400 °C in the outer regions to 6100 °C near the inner core. Because of its high temperature, modeling work has shown that the outer core is a low viscosity fluid (about ten times the viscosity of liquid metals at the surface) that convects turbulentlyEddy currents in the nickel iron fluid of the outer core are believed to influence the Earth's magnetic field. The average magnetic field strength in the Earth's outer core was measured to be 25 Gauss, 50 times stronger than the magnetic field at the surface. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core. Sulfur and oxygen could also be present in the outer core.
          This growth rate is estimated to be 1 mm per year.As heat is transferred outward toward the mantle, the net trend is for the inner boundary of the liquid region to freeze, causing the solid core to grow.

II-2-Effect on life
            Without the outer core, life on Earth would be very different. Convection of liquid metals in the outer core creates the Earth's magnetic field. This magnetic field extends outward from the Earth for several thousand kilometers, and creates a protective bubble around the Earth that deflects the Sun's solar wind. Without this field, the solar wind would directly strike the Earth's atmosphere. This could potentially have slowly removed the Earth's atmosphere, rendering it nearly lifeless, as is hypothesized for Mars.
II-3-Seismic signature
        The low viscosity of the outer core is important in seismology because low-viscosity fluids can not sustain shear stresses: their rapid deformation in response to shear stresses causes the stresses to go to zero. Therefore, s-waves attenuate completely in the outer core, and the only s-waves that appear after a wave exits the outer core do so due to splitting of p-waves into an s-wave component.

See also
              Structure of the Earth
            Iron meteorite
            Travel to the Earth's center


See more pictures about the core of the Earth